Environmental Engineering Reference
In-Depth Information
gation stages are often termed
salt component-induced
hot corrosion processes.
Some elements, such as chlorine, sulfur, and carbon, can produce such effects
depending on the deposit composition. In the cases of Na
2
SO
4
-NaCl deposits,
sulfur and chlorine are the significant elements. The importance of the element
carbon is emphasized by the fact that in fluidized-bed reactors for combustion
of coal, lime is added. The lime accepts sulfur from the coal and is converted
to CaSO
4
. Because the beds are characterized by low oxygen activity and because
of simultaneous presence of CaO, CaSO
4
, and CO
CO
2
, relatively high SO
2
activities are produced according to
CaSO
4
CO
CaO
SO
2
CO
2
(6.40)
which ultimately leads to rapid degradation of the metallic components involving
oxidation and/or sulfidation due to the reaction with SO
2
.
6.7.3 Scale Morphology
The morphology of the scales resulting from hot corrosion reactions should be
different from that associated with simple oxidation of a metal/alloy in a single
oxidant environment because oxidation and sulfidation commonly occur simulta-
neously. Investigations related to fundamentals of hot corrosion on pure metals,
binary, and ternary alloys (mostly Ni- and Co-based) have revealed widely differ-
ent scale morphologies due to varying experimental conditions [50]. Even case
histories of commercially used Ni- and Co-based alloys that have suffered such
damage under different operating conditions are reported to have revealed various
types of scale morphologies [55]. In most service cases the general consensus is
that a layer of sulfide particles forms beneath a region of porous oxide. The
sulfides have often been identified as chromium-rich, even though nickel sulfides
have also been reported in certain cases. Furthermore, the extent of internal sul-
fide layer can vary considerably depending on the service/test conditions. At
times the sulfide layer appears to be virtually nonexistent, or it may appear as a
very thin band of fine discrete sulfide particles or even as a large band of intercon-
nected sulfide particles.
In general terms, the corrosion product morphology of Ni- and Co-based su-
peralloys can be described to consist of an outer porous oxide layer, an intermedi-
ate thick layer of oxide dispersed with metal-rich fragments, and an inner layer
of fine sulfide particles in the metal matrix. The outer porous oxide layer consists
primarily of simple oxide of the base metal, i.e., either NiO or CoO depending
on the type of superalloy, and at times with some spinel. Beneath this zone, there
exists an intermediate layer comprising Ni- or Co-rich fragments in a Cr
2
O
3
ma-
trix with some spinel. Finally, the inner zone, which is in the immediate vicinity
